1. Field of the Invention
The present invention relates to a method and apparatus for handling an uplink grant, and more particularly, to a method and apparatus for handling an uplink grant received on a physical downlink control channel (PDCCH) in a user equipment (UE) of a wireless communication system, so as to avoid misusing the uplink grant for transmission.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE system, when a user equipment (UE) wants to access radio network resources, it needs to connect to nearby base stations for timing and frequency synchronization, and starts to receive system information broadcasted by the network. The UE sends an access request on a Random Access Channel (RACH) based on the received system information and parameters. There are five reasons for triggering a random access procedure: (1) Initial access request; (2) Reestablishment request; (3) Handover request; (4) Uplink data arrival; and (5) Downlink data arrival. In general, it is possible that multiple access requests from different UEs arrive at the base station at the same time. The random access procedure can be divided into two types: Contention-based, e.g. the way the UE uses to perform initial access, and Non Contention-based, e.g. the random access procedure initiated by handover with a dedicate preamble provided by a base station.
According to 3GPP specification TS 36.321 v8.2.0, the contention-based random access procedure consists of exchange of four messages: Message 1, Message 2, Message 3 and Message 4. From a standpoint of the UE, Message 1 and Message 3 are for uplink while Message 2 and Message 4 are for downlink. At first, Message 1 transmits a Random Access Preamble, randomly selected by the UE from a selected group of Random Access Preambles, to the network for requesting an uplink grant. Several UEs may use the same Random Access Preamble at the same time. Message 2, also called a Random Access Response message, carrying an uplink grant and a Temporary Cell Radio Network Temporary Identifier (Temporary C-RNTI) is then transmitted from the network to the UEs those sent the Random Access Preamble. Thus, the UEs using the same Random Access Preamble in Message 1 would receive the same uplink grant and Temporary C-RNTI from Message 2 and use the same uplink grant to transmit Message 3, so as to cause contention between the UEs. The content carried by Message 3 mainly includes uplink data and a User Equipment Identity (UE ID). Message 4, also called a Contention Resolution message, is then used for the network to resolve the contention between the UEs.
The way in which Message 4 is dealt with by the UE is based on the UE ID type carried in Message 3. The UE ID carried in Message 3 can be divided into two types: Cell Radio Network Temporary Identifier (C-RNTI) MAC control element and Common Control Channel Service Data Unit (CCCH SDU). The C-RNTI MAC control element is a MAC control element generated by MAC layer, while the CCCH SDU is generated by Radio Resource Control (RRC) layer, for performing RRC Connection Request or RRC Connection Reestablishment Request. When Message 3 including a C-RNTI MAC control element is transmitted, the UE shall start a Contention Resolution Timer and monitor whether there is a Message 4, an uplink grant received on a Physical Downlink Control Channel (PDCCH) addressed to the C-RNTI of the UE. But if the random access procedure is triggered by downlink data arrival, the PDCCH transmission addressed to the C-RNTI does not necessarily contain an uplink grant. If the UE receives a Message 4, the UE considers its Contention Resolution successful and considers itself as a contention winner; otherwise, if the UE cannot receive a Message 4 until expiry of the Contention Resolution Timer, the UE considers its Contention Resolution not successful and considers itself as a contention loser.
On the other hand, when Message 3 including a CCCH SDU is transmitted, the UE also starts the Contention Resolution Timer but uses the Temporary C-RNTI received from Message 2 to monitor PDCCH for reception of Message 4. However, the UEs using the same Random Access preamble receive the same Message 2, and obtain the same uplink grant and Temporary C-RNTI. Therefore, when the network sends a Message 4 addressed to the Temporary C-RNTI, the UEs obtaining the same Temporary C-RNTI would all receive this Message 4. In this case, the UE further compares a UE Contention Resolution Identity MAC control element decoded from the received Message 4 with the CCCH SDU carried in Message 3. If the UE Contention Resolution Identity MAC control element matches the CCCH SDU transmitted in Message 3, the UE considers its Contention Resolution successful and considers itself as a contention winner; otherwise, the UE considers its Contention Resolution not successful and considers itself as a contention loser. In addition, if the UE cannot receive a PDCCH transmission addressed to the Temporary C-RNTI until expiry of the Contention Resolution Timer, the UE also considers its Contention Resolution not successful and considers itself as a contention loser.
Please refer to FIG. 1, which illustrates a contention resolution operation of the prior art. Assume that the UE A sends a Message 3 including CCCH SDU to the network at a timing point S. Meanwhile, the UEA starts a Contention Resolution Timer, and monitors PDCCH addressed to a Temporary C-RNTI received from Message 2. Afterwards, the UE receives a Message 4 at a time point T and determines itself as a contention winner after decoding this Message 4. At this time, the UE A stops the Contention Resolution Timer, and uses the Temporary C-RNTI as its C-RNTI. Finally, the UE A sends an HARQ acknowledgement message ACK to the network, and the network allocates an uplink grant to the C-RNTI of the UE A (at a timing point U) for allowing the UE A performing uplink transmission at a timing point V.
Please continue to refer to FIG. 1. Assume that contention occurs between the UE A and the UE B, and the UE B sends a Message 3 including C-RNTI MAC control element to the network at a timing point S. Meanwhile, the UE B starts its Contention Resolution Timer, and monitors PDCCH addressed to its C-RNTI and monitors PDCCH addressed to the Temporary C-RNTI received from Message 2. The purpose to monitor PDCCH addressed to the Temporary C-RNTI is to receive an uplink grant for Message 3 adaptive retransmission. When transmission of the Message 3 fails, the network uses the Temporary C-RNTI to allocate uplink grants to the UE B for retransmission of the Message 3. In FIG. 1, the UE A has already been a contention winner at the timing point T. However, the UE B may not know that it has already become a contention loser until expiry of its Contention Resolution Timer. Thus, the UE B may still use the same Temporary C-RNTI as the UE A to monitor PDCCH, such that the UE B may receive the uplink grant allocated to the UE A at the timing point U and misuse the uplink grant for uplink transmission at the timing point V. As a result, uplink transmission of the contention winner (i.e. the UE A) cannot be successfully performed, causing waste of radio resources.
Besides, when the UE ID carried by Message 3 is CCCH SDU, misuse of the uplink grant may also occur. Please continue to refer to FIG. 1. Assume that contention occurs between the UE A and the UE C, and the UE C transmits a Message 3 including CCCH SDU to the network at a time point S. At the same time, the UE C starts its Contention Resolution Timer, and monitors PDCCH addressed to the Temporary C-RNTI received from Message 2. Generally, when the UE C receives a Message 4 at a timing point T and finds the UE ID decoded from the Message 4 does not match CCCH SDU transmitted in Message 3, the UE C determines itself as a contention loser. However, if the Message 4 cannot be successfully received or decoded before expiry of the UE C's Contention Resolution Timer, the UE C may not know itself as a contention loser and still use the same Temporary C-RNTI as the UE A to monitor PDCCH. In this situation, the UE C would receive the uplink grant allocated to the UE A at the time point U, and misuse the uplink grant for uplink transmission at the timing point V, resulting in that uplink transmission of the contention winner (i.e. the UE A) cannot be successfully performed.
As mentioned above, for the cases of Message 3 including CCCH SDU, if the UE determines itself as a contention winner after the Message 4 is successfully decoded, the UE would use the Temporary C-RNTI received from the Message 2 as its C-RNTI. However, the other UEs may not know itself has become a contention loser due to some reasons and still use the Temporary C-RNTI identical to the C-RNTI of the contention winner to monitor PDCCH, causing the uplink grant allocated to the contention winner is misused by the contention loser. As a result, the UL transmissions by the winner and losers are collided, which is the problem to be solved in the present invention.